What describes a sudden reduction in population size that reduces genetic diversity?

A sudden reduction in population size that significantly decreases genetic diversity is best described by the concept of a population bottleneck. This phenomenon occurs when a population's size is drastically reduced due to environmental events, such as natural disasters, habitat destruction, or human activity. The remaining individuals after such an event may not represent the genetic diversity of the original population, leading to a reduced gene pool. This reduction can limit the population's ability to adapt to future environmental changes and increases the likelihood of inbreeding and genetic disorders.

In contrast, genetic drift refers to the random fluctuations in allele frequencies that occur in small populations and does not necessarily involve a sudden reduction in population size. Adaptive radiation is the process where organisms rapidly diversify and adapt to different environments, rather than a decrease in population size or genetic diversity. The founder effect occurs when a small group from a larger population establishes a new population, which can lead to reduced genetic diversity, but it is distinct from a population bottleneck in that it is not the result of a sudden loss of individuals from the original population.